Table Of ContentElectron (positron) beam polarization by Compton
scattering on circularly polarized laser photons ∗
G.L. Kotkin1), V.G. Serbo1), and V.I. Telnov2)
1)Novosibirsk State University, 630090, Novosibirsk, Russia
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2)Budker Institute of Nuclear Physics, 630090, Novosibirsk, Russia
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May 14, 2002
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a
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Abstract
3
In a number of papers an attractive method of laser polarization of electrons
v
9 (positrons) at storage rings or linear colliders has been proposed. We show that
3 these suggestions are incorrect and based on errors in the simulation of multiple
1
Compton scattering and in the calculation of the Compton spin-flip cross sections.
5
0 We argue that the equilibrium polarization in this method is zero.
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0
/
h 1 Introduction
p
-
p Experiments at SLC have shown great potential of polarized e± beams for investigation
e
h of new physical phenomena. In all projects of future e+e−, e−e−, γγ and γe linear
: colliders [1], electron and positron beams with high degree of polarization are foreseen,
v
i though this is not an easy task. That is why any new methods for obtaining polarized e±
X
beams are very welcome.
r
a
There are two well–known and recognized methods for production of polarized beams
forlinear colliders. Inthefirst method, electronbeamswithapolarizationof80%(maybe
even higher) are obtained using photoguns [2]. Another method of polarization, suitable
both for electron and positron beams, is based on a two-step scheme [3]. At the first
step, the unpolarized electron beam passes through a helical undulator (or collides with
circularly polarized laser light) and produces photons with maximum energy of about
30÷50 MeV. These photons have a high degree of circular polarization in the high energy
partofthespectrum. Thenthesephotonspassthroughathintungstentargetandproduce
e+e− pairs. At the maximum energies, these particles have a high degree of longitudinal
polarization. The expected polarization of electron and positron beams in this method is
45÷60 % [4, 5].
Additionaly, inanumberofpapersanewattractivemethodforproductionofpolarized
electron1 beams, based on the process of multiple Compton scattering of ultra-relativistic
∗
Talk at 9-th Intern. Workshop on Linear Colliders (LC02), Feb. 4-8, 2002, SLAC, Stanford, USA.
To be published in Phys. Rev. ST Accelerators and Beams.
1This method is applicable both for electrons and positrons, for brevity we will omit “positrons”
hereafter
1
electrons on the circularly polarized laser photons, was discussed. Here one should distin-
guish between two possibilities: polarization of the beam at the cost of a loss in intensity
and polarization without loss of intensity. The latter case is the subject of the present
paper.
It is well known that in Compton scattering the electrons of different helicities are
knocked out of the beam differently. As a consequence, after removal of the scattered
electrons, the electron beam can get a considerable polarization at the expense of a con-
siderable loss of its intensity. A detailed consideration of this method was given in [8].
Though the basic idea of this method is correct, it was not used in practice because the
losses in intensity during the polarization process are too large.
In this paper we critically consider another proposal of beam polarization which is
based on multiple Compton scattering of electrons on laser photons without loss of inten-
sity. It implies that during a single Compton scattering the energy loss of an electron
is small and the electron scattering angle is small as well; therefore, the scattered elec-
trons remain in the beam. It means that the electron energy is of the order of 1 GeV
for laser light with a photon energy of about 1 eV. Such proposals were given in pa-
pers [9, 10, 11, 12]. They were cited in a number of papers (see Refs. [13], for example)
and attracted attention at the Snowmass 2001 conference. It was suggested to implement
this method either on a storage ring (where the electron beam collides with laser beams
many times at a single point) orat a linear collider (where the electron beamwould collide
with laser beams at several points) with reacceleration between them.
The theoretical consideration of the process of multiple Compton scattering in pa-
pers [9]–[12] is based on two different approaches. The first approach exploits the fact
that the scattered electrons are polarized even if the initial electron beam is unpolar-
ized. Since the scattered electrons do not leave the beam, multiple collisions with laser
photons would appear to lead to a growth of the mean electron beam polarization. The
quantitative consideration of this idea in papers [10, 11] resulted in
• Conclusion 1 [10, 11]. A longitudinal polarization of electrons (positrons) up to
100 % can be achieved in a relatively short time.
In Sect. 3 we explain the origin of the mistake that lead to “Conclusion 1”. Briefly
speaking, in simulation of multiple Compton scattering one should not only consider the
multiple Compton scattering of the same electron but also take into account the fact that
the polarization of unscattered electrons changes in the laser wave as well. The correct
simulation procedure for multiple Compton scattering (Sect. 2) leads to zero polarization
of the final electron beam.
In the second approach, only the equilibrium polarization of the electron beam af-
ter multiple passes through the laser beam was considered. Let w and w be the
+− −+
probabilities for Compton scattering with a given electron spin-flip. It is not difficult to
show (see Sect. 2.2) that the electron beam gets the maximal equilibrium polarization
ζ(f) = (w −w )/(w +w ). The corresponding probabilities have been calculated
z −+ +− −+ +−
in papers [9, 12] with the following
• Conclusion 2 [9, 12]. The longitudinal polarization of electrons (positrons) as large
as 62.5% can be achieved.
In Sect. 4 we show that Conclusion 2 is due to an error in the calculation of the Comp-
ton spin-flip cross sections. The error is connected to the incorrect transition between
2
the collider frame (CF) and the rest frame of the initial electron (RFIE). The correct
result corresponds to w = w , therefore, the discussed process of laser polarization is
+− −+
impossible.
Below, in Sect. 2, we present a set of formulae for Compton scattering that takes
into account the particle polarization from Ref. [6], as well as a short description of
the simulation procedure for the multiple Compton scattering from Ref. [7], which are
useful for quantitative consideration of this method. Here we show that scattering of
laser photons do not lead to polarization of electron beams, in fact they lead only to
depolarization. In Sect. 3 and 4 we explicitly show the origin of mistakes in the previous
papers on this subject.
2 Polarization of final electrons
2.1 Polarization of final electrons in single Compton scattering
We consider the basic Compton scattering
e(p)+γ(k) → e(p′)+γ(k′) (1)
in the CF, in which an electron with energy E ∼ 1 GeV collides a head-on with a laser
photon of energy ω ∼ 1 eV.
LetusintroducesomenotationrelatedtotheComptonscattering(1)inCF.Wechoose
the quantization axis (z-axis) along the initial electron momentum p (i.e., anti-parallel
to the laser photon momentum k). Let P = +1 be the mean helicity of the circularly
c
polarized laser photons, ζ= (0, 0, ζ ) and ζ′ = (0, 0, ζ′) be the polarization vectors of the
z z
electron in the initial and final states. It is convenient to describe the Compton scattering
by the invariants
2pk 2pk′ 2p′k
x = , y = = . (2)
m2 pk pk
e
In CF, we have
4Eω ω′
x = , y = . (3)
m2 E
e
The maximum energy of scattered photons is
x
max{ω′} = y E, y = . (4)
m m
x+1
For E ∼ 1 GeV and ω ∼ 1 eV, the value of x ∼ 0.015, therefore hereafter we assume
x ≪ 1, E −E′ = ω′ ≪ E. (5)
InRFIE,theenergy ofthelaser photonxm /2is smallincomparisonwiththe electron
e
mass: xm /2 ≪ m ; and therefore the transverse momenta are small as well:
e e
|p′ | = |k′ | < xm /2 ≪ m . (6)
⊥ ⊥ e e
As a result, the electron scattering angle in CF is very small
|p′ | xm 2ω
θ ≈ ⊥ < e ≈ ∼ 4×10−6. (7)
e p′ 2E m
z e
3
The Compton cross section in the collider frame for the above conditions has been
given in [6]:
dσ πr2
= e [(1+ζ ζ′)F +(ζ +ζ′) P F +ζ ζ′ F ] , (8)
dy x z z 1 z z c 2 z z 3
1 y(2−y)c y2s2
F = +c2 −y, F = , F = − ,
1 2 3
1−y 1−y 1−y
where r is the classical electron radius and
e
y
s = 2 r(1−r), c = 1−2r, r = (9)
(1−y)x
q
(in RFIE, the invariant s = sinϑ and the invariant c = cosϑ, where ϑ is the photon
scattering angle).2
2.2 Balance equations
If we do not take into account the transverse polarization of electrons, we can describe the
electron beam as a mixture of N electrons with ζ = +1 and N electrons with ζ = −1.
+ z − z
In this case the mean value of the longitudinal electron polarization is
N −N
+ −
ζ = . (10)
z
N +N
+ −
When the electron beam travels the path dz in the laser beam with the laser photon
density n (z), the change of numbers N (z) is given by the balance equations derived by
L ±
the following simple consideration. A reduction of the number of electrons with ζ = +1
z
in the beam is determined by the quantity N (z)(dw +dw ), where
+ ++ +−
dwζzζz′ = 2σ(ζz, ζz′)nL(z)dz (11)
is the probability that an electron with a certain ζ is scattered on the path dz with the
z
transition to a certain ζ′ (here we assume also a certain P = +1). The coefficient 2 is
z c
due to the fact that the electron and the laser photon travel towards each other with
the speed of light. On the other hand, the sum N (z)dw +N (z)dw represents the
+ ++ − −+
number of scattered electrons with ζ′ = +1.
z
As a result, the total change of the number of electrons with ζ = +1 is equal to
z
dN (z) = −N (z)(dw +dw )+N (z)dw +N (z)dw (12)
+ + ++ +− + ++ − −+
= −N (z)dw +N (z)dw
+ +− − −+
and, similarly,
dN (z) = −N (z)dw +N (z)dw . (13)
− − −+ + +−
Since in the considered method the scattered electrons remain in the beam, the sum
N +N = N does not change:
+ − e
dN (z)+dN (z) = 0, (14)
+ −
2Strictly speaking, Eq. (8) given in [6] is valid for the case when ζz′ is the projection of the vector ζ′
onthemomentumofthefinalelectronp′. However,theprojectionsofζ′ onp′ andonthez-axiscoincide
in our approximationdue to the very small value of the electron scattering angle in CF (7).
4
while the mean longitudinal polarization (10), generally speaking, changes as
N dζ = dN (z)−dN (z) = −2N (z)dw +2N (z)dw . (15)
e z + − + +− − −+
Balance equations (12)–(15) are simplified in two particular cases. First, if the initial
electron beam is unpolarized, N (z) = N (z) = N /2, it gets (after traveling the path
+ − e
dz) the polarization
dζ = dw −dw . (16)
z −+ +−
Second,letusconsidertheequilibriumpolarizationoftheelectronbeam(whichisachieved
after multiple passing of the electron beam through the laser beam). In this case, dN =
+
dN = 0 or
−
N dw = N dw . (17)
+ +− − −+
From this equation one obtains the equilibrium polarization degree of the electron beam
w −w
ζ(f) = −+ +− (18)
z w +w
−+ +−
where w = dw istheprobability forthewholepathinthelaser beam. Thequantity
±∓ ±∓
w −w is proportional to the difference of the Compton spin-flip cross sections (with
+− −+ R
a certain P = +1 and with the summation over spin states of the final photon):
c
∆σ = σ(ζ = +1,ζ′ = −1)−σ(ζ = −1,ζ′ = +1), (19)
z z z z
Therefore, in both of these cases the crucial question is whether ∆σ is equal to zero or
not equal to zero.
In papers [9, 12] it was claimed that the difference
∆σ 6= 0, (20)
the process of laser polarization of particles is possible and ζ(f) = 5/8. In contrast, using
z
formulas (8) we immediately obtain
∆σ = 0 (21)
and, therefore, thepolarizationiszero.3 The originoftheincorrect result (20) isexplained
in Sect. 4.
Taking into account that dw = dw , we can rewrite Eq. (15) in the form
+− −+
dζ
z = −4σ(ζ = +1, ζ′ = −1)n (z)dz (22)
ζ z z L
z
from which it follows that the polarization |ζ | is reduced after traveling the path dz.
z
Note that the result (21) is due to the specific structure of Eq. (8): the coefficient in
front of ζ P in this equation precisely coincides with the coefficient in front of ζ′P .
z c z c
3The similar remark about the equal spin-flip probabilities in CF was given (without details) in
Ref. [14].
5
2.3 A scheme for simulation of multiple Compton scattering
In some problems such as conversion of electrons to photons at photon colliders, laser
cooling, etc., it is necessary to calculate beam parameters after multiple acts of Compton
scattering. Let an electron beam traverses a region where laser light is focused. It is clear
that the energies of these electrons as well as their polarizations vary due to Compton
scattering.
However, when the electron passes through the laser beam, the polarization varies also
for those electrons which conserve their energies and directions of motion (unscattered
electrons). This effect is due to the interference of the incoming electron wave and the
electron wave scattered at zero angle. The change in the electron polarization depends
not only on the Compton cross section but on the real part of the forward Compton
amplitude as well. Such an effect was considered in Ref. [7].
Both of these effects should be taken into account in simulation of multiple Compton
scattering. It can be taken into account in the following way. The electron state is defined
by the current values of its energy E, the direction of its momentum (along the z -axis)
and its mean polarization vector ζ. The probability to scatter on the path dz is equal to
dw = 2σ(E, ζ )n (z)dz, (23)
z L
where σ(E, ζ ) is the total cross section of the Compton scattering process. Then, as
z
usual, one can simulate whether the scattering takes place on this path dz or not.
If the scattering does take place, then, using known formulae for the Compton cross
section in CF (see Ref. [6]), one can calculate a new value of the electron polarization
vector ζ(f) and other parameters.
If the scattering does not occur, one still has to change the electron polarization
vector.4 The variation of electron polarization in the laser wave for a general case was
considered in [7]. Following that paper, the change of the electron polarization vector of
the unscattered electron is
dζ = (Rζ +Iζ ζ )P 2πr2n dz, (24)
x y z x c e L
dζ = (−Rζ +Iζ ζ )P 2πr2n dz, (25)
y x z y c e L
dζ = −I(1−ζ2)P 2πr2n dz, (26)
z z c e L
where the functions I = I(x) and R = R(x) are equal to:
2 x/(1+x y(2−y)c
I = dy = (27)
x 1−y
Z0
2 2 5 1 1
= 1+ ln(x+1)− + − ,
x "(cid:18) x(cid:19) 2 x+1 2(x+1)2#
4Thenecessityofthisstepcanalsobeseenfromthefollowingconsideration. ThevalueoftheCompton
cross section depends on polarizations of electron and laser beams. If the electron beam was initially
unpolarized, then, after the Compton scattering of one electron, the rest (unscattered) part of the beam
gets some polarization (see Sect. 2.2). That is just because electrons with different polarizations have
different scattering probabilities. In other words, the laser beam “selects” preferably electrons with a
certainpolarization. Inparticular,equation(26)forthelongitudinalpolarizationcanbe obtainfromthe
balance equations discussed above.
6
2 2 2
R(x) = 1− F(x−1)− 1+ F(−x−1)−
πx x x
(cid:20)(cid:18) (cid:19) (cid:18) (cid:19)
2x3lnx x 2π2
− + − , (28)
(x2 −1)2 x2 −1 3x #
with
x
ln|1+t|
F(x) = dt (29)
t
Z
0
being the Spence function.
Nowwecancalculatethetotalchangeoftheelectronbeampolarizationonthedistance
dz in the laser target by the means of simulation.
Iftheinitialelectronisunpolarized(ζ = 0)andthelaser photoniscircularlypolarized
z
(P = +1), then from Eq. (8) we have
c
1
σ = σ +πr2I(x)ζ′ , (30)
2 unpol e z
h i
where σ is the Compton cross section for unpolarized beams. Therefore, the scattered
unpol
electron becomes polarized after the first scattering, and its mean degree of polarization
is
πr2I(x)
ζ(f) = e . (31)
z σ
unpol
The total number of scattered electrons on the path dz in the laser target is
dN = 2σ N n dz. (32)
e unpol e L
This means that the sum of polarizations of the scattered electrons along the z-axis:
ζ(f)dN = 2πr2I(x)N n dz. (33)
z e e e L
On the other hand, the unscattered electrons become polarized in accordance with
Eq. (26), and their sum of polarizaions along the z-axis
dζ N = −2πr2I(x)N n dz, (34)
z e e e L
i.e. it completely compensates the above polarization of the scattered electrons. So, the
electron beam remains unpolarized. This coincides with our result obtained in Sect. 2.2
where only the equilibrium final state was considered.
At the end of this subsection we note the following. The components of the vector
ζ define the parameters of the polarization density matrix of an electron, among them,
ζ is related to the diagonal matrix elements while ζ and ζ determine the off-diagonal
z x y
matrix elements. If one is interested in the longitudinal electron polarization only (as
in the present paper), it is sufficient in simulation to use Eq. (26) connected with the
“occupancy” numbers N . In the general case one should use the whole set of Eqs. (24)–
±
(26).
7
3 Remark on Conclusion 1
Now we are ready to show the origin of the error in Conclusion 1. Let us describe
the procedure of the naive simulation of the multiple Compton scattering. We consider
the case when in the CF the polarization vectors of the initial and final electrons have z-
components only and the parameter x is small. The corresponding Compton cross section
with an accuracy up to the terms of the order of x can be easily obtained from (8):
4 x
σ = πr2 (1−x)(1+ζ ζ′)− P (ζ +ζ′) . (35)
3 e z z 4 c z z
(cid:20) (cid:21)
If the initial electron is unpolarized (ζ = 0) and the laser photon is circularly polarized
z
(P = +1), then
c
4 x
σ = πr2 1−x− ζ′ , (36)
3 e 4 z
(cid:20) (cid:21)
i.e. the cross section is somewhat larger for ζ′ = −1 than for ζ′ = +1. Therefore,
z z
the scattered electron becomes polarized after the first scattering and its mean degree of
polarization is
x
ζ(f) = − . (37)
z 4
Repeating this procedure, one can find that after N collisions the electron polarization is
N
ζ(f) = − , (38)
z (4/x)+N
which can reach 100% for N ≫ 4/x. This fact is the basis for Conclusion 1.
This “polarization” is not connected with the electron spin-flip, it is due to some
differences in the cross sections: the polarized laser beam selects electrons with a certain
(in our case, negative) polarization. But such a naive simulation of the multiple Compton
scattering is incorrect because it does not take into account the fact that unscattered
electrons become polarized in the opposite direction. The correct procedure for this
simulation is described in the previous section and leads to zero polarization.
4 Remark on Conclusion 2
In Sect. 2.1 we have shown that the equilibrium polarization of electrons in the discussed
method is zero. Below, we show the origin of the mistake that led to Conclusion 2. We
remind that our result (21) has been obtained in the CF. To the contrary, the authors
of Conclusion 2 had obtained their result (20) in the RFIE. Below we demonstrate how
to obtain our result in RFIE and show that the error in Conclusion 2 is connected with
inaccurate transition from CF to RFIE.
In our consideration, we use the electron polarization vectors5 ζ and ζ′, which in CF
have the forms
ζ = (0, 0, ±1), ζ′ = (0, 0, ∓1). (40)
5They determine the electron-spin 4-vectors a and a′ as
ζp ζp ζ′p′ ζ′p′
′ ′ ′
a= , ζ+p , a = , ζ +p . (39)
me me(E+me) me me(E′+me)
(cid:18) (cid:19) (cid:18) (cid:19)
8
It is not difficult to show that in RFIE the vector ζ has the same form, but the vector ζ′
has another form:
p′ k′ (p′ )2
ζ′ = ∓ ⊥ = ± ⊥ , ζ′ = ∓ 1− ⊥ ≈ ∓1, (41)
⊥ me me z 2m2e !
since transition from CF to RFIE corresponds to a boost along the vector p, but not
along the vector p′. To prove (41), it is sufficient to consider the transverse component
of the electron-spin 4-vector a′ in the CF and in the RFIE. In the CF, one has
p′ p′
a′ = ∓p′ z ≈ ∓ ⊥ , (42)
⊥ ⊥ m (E′ +m ) m
e e e
since in the CF we have p′/(E′ +m ) ≈ 1. In RFIE the same polarization vector is
z e
ζ′p′
a′ = ζ′ +p′ ≈ ζ′ , (43)
⊥ ⊥ ⊥ m (E′ +m ) ⊥
e e
since in RFIE we have |p′|/m ≪ 1.
e
The needed Compton cross section in RFIE can be found in the textbook [15] (see
Eqs. (87,22) and (87,23)):
dσ r2 ω′ 2
= e [F +fζP +gζ′P +G ζ ζ′ ] . (44)
dΩ 4 ω ! 0 c c ik i k
From this we get the following result:
d∆σ r2 ω′ 2 k′
= e f −g +g ⊥ . (45)
z z ⊥
dΩ 2 ω ! " me #
Before proceeding, we note that the momenta of the initial and final photons in RFIE are
k = (0,0,−ω), k′ = (k′ , −ω′cosϑ) (46)
⊥
where ϑ is the photon scattering angle (the direction of the z-axis is along the vector
(−k)). Using Eq. (87,23) from [15] and taking into account the terms of the second order
in ω/m we obtain
e
ω −ω′cosϑ ω +ω′ ω 2
f −g = sin2ϑ ≈ (1−cosϑ) sin2ϑ, (47)
z z m 2m+ω −ω′ m
e (cid:18) e(cid:19)
k′ (k′ )2 ω 2
g ⊥ = − ⊥ (1−cosϑ) sin2ϑ ≈ − (1−cosϑ) sin2ϑ. (48)
⊥
m m2 m
e e (cid:18) e(cid:19)
As a result, we get
d∆σ
= 0, (49)
dΩ
which is in agreement with the conclusion (21) in CF.
The wrong conclusion (20) was obtained because the same form (40) was used for the
vector ζ′ both in the CF and in the RFIE. It is equivalent to omitting the last term in
the square bracket in Eq. (45).
Thus, the calculations, performed in CF as well as in RFIE, give us the same result
(21). We, therefore, conclude that the claim (20) is based on an inaccurate transition
from CF to RFIE.
9
5 Summary
We have shown that the multiple Compton scattering of electrons on circularly polarized
laser photons at usual storage rings or linear accelerators does not lead to polarization
of electron beams. Statements by some authors about obtainability of high degrees of
polarizationareexplained by mistakes in their calculationprocedures. Wehaddiscussions
with E.G. Bessonov and A.P. Potylitsyn, and they agreed with our criticism.
In this paper we have considered the linear Compton scattering (the scattering of an
electron on a single laser photon). It is technically possible to realize conditions which
correspond to the nonlinear Compton scattering (the scattering of an electron on several
laser photons). The effective cross section for the nonlinear Compton scattering from
Ref. [16] has the same specific structure as Eq. (8) but with much more complicated
functions F . From this, one can easily obtain the result (21), which means that the
1,2,3
equilibrium polarization of electrons is zero in the the case of the nonlinear Compton
scattering as well.
One additionalremark. There is no polarizationof the electron beam as a whole in the
considered scheme, however, it does not close the possibility to use lasers for polarization
of electron beams in other schemes. For example, it has been shown in Ref. [14] that
using specially arranged spin-orbit coupling in damping rings (by adding a solenoid), a
polarization of about 60 % may be reached. This method is based on the difference in
the Compton cross sections for electrons with different values of their helicities, on the
fact that scattered electrons have lower energy compared to unscattered electrons, and
on dependence of the spin precession angle on the electron energy. This method is not
simple, and is too slow for preparation of beams for linear colliders.
Acknowledgement
We are very grateful to E. Bessonov, R. Brinkmann, V. Katkov, A.P. Potylitsyn, E.L. Sal-
din, A.N. Skrinsky and V. Strakhovenko for useful discussions. This work is supported in
part by INTAS (code 00-00679), RFBR (code 00-02-17592 and 00-15-96691) and by St.
Petersburg grant (code E00-3.3-146).
References
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Collider Working Group, T. Abe et al., SLAC-R-570, May 2001.
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X-ray laser laboratory. Technical design report, 6 parts, Ed. F.Richard et al., DESY
2001-011, ECFA 2001-209, March 2001.
(JLC) N. Akasaka et al., ACFA Linear Collider Working group (K. Abe et al.) KEK-
REPORT-2001-11, hep-ph/0109166.
[2] J.E. Clendenin et al., AIP Conf. Proc. 421, p.250.
[3] V.E. Balakin and A.A. Mikhailichenko, Preprint INP 79-85, Novosibirsk, 1979.
[4] TESLA TDR, Part. 2, see [1].
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